1. Field of the Invention
The present invention is generally directed to vehicle suspension systems, and in particular, to a control method for controlling vehicle suspension systems. The present invention is described with respect to suspension systems developed by the applicant. It is however to be appreciated that the control method is also applicable for other vehicle suspension systems.
2. Description of the Background Art
The Applicant has developed a suspension system disclosed in Australian Patent Application No. 23664/92, details of which are incorporated herein by reference. This suspension system incorporates double acting rams for each vehicle wheel. The upper and lower chambers of each front wheel ram is respectively interconnected with the lower and upper chambers of the diagonally opposite rear wheel ram to provide a fluid circuit thereof. A corresponding fluid circuit is provided with the other front wheel ram and rear wheel ram. The two fluid circuits are interconnected by a load distribution unit which is arranged to maintain at least substantially equal pressure in the two fluid circuits.
In the Applicant""s International Application No. PCT/AU94/00646, there is disclosed a control arrangement for the above described suspension system incorporating a control arrangement for maintaining the vehicle in a position at least substantially parallel to the plane of the ground upon which the vehicle is traversing, details of which are also incorporated herein by reference. The control arrangement includes sensor means adapted for generating a signal indicative of the extension of each wheel ram, and therefore the position of each of the wheels relative to the vehicle body. The control arrangement then determines the average height of the vehicle body at selected locations between respective pairs of orthogonally adjacent wheels. Adjustment means are provided to adjust the quantity of fluid in the fluid circuits communicating with the rams of the said orthogonally adjacent wheels to establish the preset datum height between the orthogonally adjacent wheels and the vehicle body at the said selected locations.
The wheel rams are described as being adjusted in orthogonal pairs to avoid disturbing the position of the load distribution unit as the fluid quantity in each circuit is adjusted in the above application. However, because of the interconnection between the wheel rams, adjustment of the average height between one orthogonal wheel pair can produce a change in the average height between other orthogonal wheel pairs which can result in the overshooting of the average height between the other orthogonal wheel pairs past the desired height. The control arrangement must therefore adjust the average height between each of the orthogonal wheel pairs over a number of steps in an iterative process to progressively minimise this height error until the required average height is reached between each pair. This means that the fluid must be pumped and released from each of the fluid circuits over a number of steps to bring the vehicle body to a position parallel to the plane of the ground.
The primary problem of such an iterative process is that a large amount of fluid must be cycled to and from the circuits with fluid being pumped into and removed from each fluid circuit over a number of cycles until the vehicle reaches it is final position. Therefore, much energy and time is wasted in this iterative process.
It is therefore advantageous to have a control method which allows for faster adjustment of the vehicle body position.
With this in mind, the present invention provides a method of controlling a vehicle suspension system for a vehicle body supported by a plurality of spaced apart support means arranged in at least generally diagonally opposite pairs, the vehicle suspension system.including adjustment means for adjusting the position of each of the support means relative to the vehicle body, sensor means adapted to generate a signal indicative of the position of each of the support means, and control means arranged to receive said signals, the method including determining the position of each support means, determining a diagonal average for each pair of diagonally opposite support means, the diagonal average being the average of the position of one support means and the position of the diagonally opposite support means, wherein the required position for each support means is a function of at least two said diagonal averages, the position of each support means being adjusted on the basis of the said at least two diagonal averages.
As the control method determines the required position of each wheel individually, this provides for a faster adjustment of the vehicle position.
The control method preferably includes determining the diagonal average of the front left and back right wheels of the vehicle, and the diagonal average of the front right and back left wheels of the vehicle. The control method may then determine the difference between the diagonal averages to provide the xe2x80x9ctotal articulationxe2x80x9d of the vehicle. The term xe2x80x9carticulationxe2x80x9d refers to the situation where the first pair of the diagonally opposite wheels moves together in a common direction and the position of the second pair of wheels remains unchanged or moves together in an opposing common direction to the first pair thereof such that there is relative displacement between each pair of diagonally opposite wheels. As well as providing an indication of the degree of articulation of the wheels of the vehicle, the determination of the total articulation of the vehicle also indicates the flatness of the terrain upon which the vehicle is supported. When the vehicle position is on a flat surface, then the respective diagonal averages would be the same showing that the vehicle is not in articulation. When the vehicle is resting on or travelling over an undulating surface, the respective diagonal averages can differ showing that the vehicle is in articulation.
A xe2x80x9cheight set pointxe2x80x9d may be selected, this height set point being the desired height of the vehicle. A single overall height set point may be selected. Alternatively, the height set point may consist of a front height set point and a back height set point to allow pitch attitude adjustment of the system. These height set points may be manually selected by the driver of the vehicle and/or may be automatically selected by the control means in dependence on predetermined conditions. The height set points of the vehicle may for example be adjusted as a function of the vehicle speed. The height set points may be lowered when the vehicle is above a predetermined speed. The vehicle will generally only be moving at high speeds when the vehicle is not in an off road location where a lower height set point is desired.
The control method may also take into account any difference in the roll stiffness between the front and back of a vehicle which will vary between different models of vehicles. The ratio of the roll stiffness of the front of the vehicle and the total roll stiffness of the vehicle is referred to as the xe2x80x9cRoll Splitxe2x80x9d or xe2x80x9cRoll Couple Distributionxe2x80x9d of the vehicle which will be a constant value for that vehicle. When the roll stiffness of the front and rear of the vehicle is the same, the Roll Split constant would be 0.5.
The wheel positions may preferably be measured on the basis of a calibrated scale to take into account differences such as in the relative degree of extension between the front and rear wheels. Each wheel position may for example be measured on a scale between 0 and 1. A wheel position may be designated to be at 0 when the wheel is fully retracted, i.e at or near the bump stop, and may be designated to be at 1 when at or near full extension. The other wheels may be calibrated on the same scale taking into account possible different wheel travel positions. It is alternatively possible to measure the actual extension of the wheels for the determination.
The control method may therefore determine the required position or the wheel set points of each wheel as follows:
FWSP=FSPxc2x1(1.0xe2x88x92RS)xc3x97(D1xe2x88x92D2)
BWSP=BSPxc2x1RSxc3x97(D1xe2x88x92D2)
where
FWSP is the wheel set point for the front wheels,
BWSP is the wheel set point for the back wheels,
FSP is the front height set point of the vehicle,
BSP is the back height set point of the vehicle,
RS is the Roll Split constant for the vehicle, and
D1, D2 are the respective diagonal averages for each pair of diagonally opposite wheels.
The position of each wheel relative to the vehicle body may be continuously adjusted on the basis of the above noted control method. Alternatively, the control method can be initiated at regular or irregular intervals, and the wheels adjusted at that time. The periods between initiation of the control method could for example be as little as 10 milliseconds which provides sufficient time for the valves controlling the fluid flow to and from the fluid circuits to open and close.
When the vehicle is moving, it may not be necessary to continually adjust the position of each wheel to wheel set points determined by the control method. For example, the average wheel position of each wheel over a preset period could be determined and the wheel set points determined on the basis of the average wheel positions. It is alternatively possible to do a series of wheel set point determinations over a preset period to thereby obtain the average wheel set points over that preset period. The average wheel positions will also be determined over that period and the wheel positions adjusted on the basis of the average wheel set points and average wheel positions. However, at start up, where significant changes in the load distribution in the vehicle can arise as a result of people and other loads being placed within the vehicle, the actual instantaneous wheel set point of each wheel determined by the control method can be used to adjust the wheel positions to thereby quickly position the vehicle in response to those load changes.
The control means may also determine the rate of change of the vehicle speed. Therefore, if the rate of change of the speed is above a certain rate, then the control method will not adjust the wheel positions. This will take into account when the vehicle is braking or accelerating when such adjustment of the wheel positions are not required or desired.
In another arrangement a door sensor may be provided to sense the closing of the door so that the control method will only be actuated after the occupants are within the vehicle.
In the vehicle suspension system described in international application no. PCT/AU94/00646, a load distribution unit is provided to equalise the fluid pressures between the two fluid circuits to thereby at least substantially distribute the load between the wheels of the vehicle. One of the described embodiments of the load distribution unit includes two separate displaceable piston assemblies mounted in a series arrangement and moveable along a common axis, each piston assembly including a piston rod and a piston supported thereon. It is however to be understood that although the following description will refer to the above noted arrangement, the housings are in practice located adjacent to each other such that the piston assemblies move along adjacent parallel axes. Each piston is slideably mounted within a respective housing and separates the chamber into inner and outer chambers, these chambers being in communication with the conduits connecting the wheel rams. A xe2x80x9cfront bumpxe2x80x9d chamber is also provided between the housings for accommodating an inner end of each piston rod. Respective xe2x80x9cback bumpxe2x80x9d chambers are also provided on the opposing side of each housing, each back bump chamber accommodating a respective outer end of one of the piston rods. Fluid accommodated within the front bump chamber is compressed when both the front wheels of the vehicle go over a bump whereas the fluid in the back bump chambers are compressed when both the back vehicle wheels goes over a bump. It is noted that this arrangement can be reversed by changing the interconnection of the load distribution unit with the wheel rams. Addition or removal of fluid from the front bump and back bump chambers control the position of each piston relative to its respective housing. It should be noted that in alternative embodiments of the load distribution unit, the pistons are supported on a single piston rod. Alternatively, a solid resilient member can be provided between the two piston rods. In both of these embodiments, there is little to no relative displacement between the pistons, and the pistons move together as a unit.
When the wheels are in articulation, the piston assemblies move in tandem and there is no relative movement between them. However, when a load is placed in the back or the front of the vehicle, the piston assemblies will move respectively closer or further apart so that there is relative movement between the piston assemblies.
Each piston assembly is ideally located at an intermediate position when the wheels are not in articulation, and move away from that intermediate position when a load is placed on one end of the vehicle and/or when the wheels are in articulation. At this intermediate position, the pistons may be located approximately half way between the end walls of each housing to provide maximum clearance therebetween to thereby provide unimpeded movement of the pistons. The pistons will move away from their intermediate position when the wheels are in articulation but should ideally return to that intermediate position when the wheels are not in articulation. However, due to non-linearities in the sensor means and so on resulting in an error in the position of, the piston assemblies from the intermediate position even when the wheels are not in articulation which can possibly adversely affect the operation of the load distribution unit. Sensor means are therefore also provided to determine the position of each of the piston assemblies and thereby enable adjustment of the position of the piston assemblies, for example by controlling the fluid flow into and out of that load distribution unit. It is to be noted that in the alternative embodiments of the load distribution unit where the pistons move as a unit, only one sensor means is required.
The control method according to the present invention therefore also preferably includes controlling the position of the piston assemblies of the load distribution unit. The position of each piston may be measured on the basis of a calibrated scale between 0 and 1. The wheels of the vehicle may be articulated in one direction physically or in a computer model of the suspension system until the difference between the respective diagonal averages is equal to 1.0 (i.e D1xe2x88x92D2=1.0) showing that the wheels are fully articulated in one direction. The position of the piston at full articulation is designated to be at 0. The wheels are then articulated in the opposing direction until the difference between the respective diagonal averages is equal to xe2x88x921.0 (i.e D1xe2x88x92D2=xe2x88x921.0) showing that the wheels of the vehicle are fully articulated in the opposing direction. The piston position may therefore be measured and determined on the basis of a scale from 0 to 1.
The load distribution unit will preferably need to be xe2x80x9ccentredxe2x80x9d such that the pistons will be at the required intermediate positions when the vehicle is not in articulation. To this end, the position of each piston will preferably be displaced to satisfy the following equation;   PAV  =                    P1        +        P2            2        =    0.5  
where
PAV is the average position of the pistons, and P1, P2 are the respective positions of each piston.
The pistons of the load distribution unit are preferably initially centred to the required position during, for example periodic servicing of the vehicle. This may be achieved by adding or removing fluid from the front bump chamber of the load distribution unit to maintain a predetermined regulating pressure therein. At the same time, the pistons of the load distribution unit may be centred by adding or removing fluid from the back bump chambers of the load distribution unit so that the average position of the pistons relative to the end walls of their respective housing is 0.5. This will allow correction of the positioning of the pistons even when the wheels are in articulation during the centring procedure because the average position of the pistons are corrected.
Once the load distribution unit is adjusted as above, a fixed amount of fluid may be maintained within the front bump chamber. During subsequent operation of the vehicle, the load distribution unit can be centred by adding or removing fluid from the back bump chambers only.
The set points of each of the pistons can be determined on the basis of the following equation;   LDUSP  =      0.5    ±                  D1        -        D2            2      
where
LDUSP is the set point of each piston of the load distribution unit, and D1, D2 are the respective diagonal average of each pair of diagonally opposite wheels, the diagonal averages being expressed on a scale between 0 and 1.
This determination takes into account the total articulation of the wheels and determines when the pistons are not in the required position.
The pistons may then be displaced to the required position by adding fluid to the inner chamber and removing fluid from the outer chamber of one housing and adding fluid to the outer chamber and removing fluid from the inner chamber of the opposing housing to move the pistons in tandem in one direction while maintaining the relative distance between them. The pistons can be moved in tandem in the other direction by reversing the fluid flow.
It should be noted that the above determination of the load distribution unit set points is only required for one piston in the alternative embodiments where the pistons move together as a unit.
The control means may be an electronic control unit. It is however also possible for the control means to comprise analogue or mechanical means.
It will be convenient to further describe the invention by reference to the accompanying drawings which illustrates possible control methods of the invention. Other embodiments of the control method of the invention are possible, and consequently the particularity of the accompanying drawing is not to be understood as superseding the generality of the preceding description of the invention.